NO2-Sensitive SnO2 Nanoparticles Prepared Using a Freeze-Drying Method

The n-type semiconductor SnO2 with a wide band gap (3.6 eV) is massively used in gas-sensitive materials, but pure SnO2 still suffers from a high operating temperature, low response, and tardy responding speed. To solve these problems, we prepared small-sized pure SnO2 using hydrothermal and freeze-drying methods (SnO2-FD) and compared it with SnO2 prepared using a normal drying method (SnO2-AD). The sensor of SnO2-FD had an ultra-high sensitivity to NO2 at 100 °C with excellent selectivity and humidity stability. The outstanding gas sensing properties are attributed to the modulation of energy band structure and the increased carrier concentration, making it more accessible for electron exchange with NO2. The excellent gas sensing properties of SnO2-FD indicate its tremendous potential as a NO2 sensor.


Introduction
With the development of industry, global environmental pollution has become increasingly serious, and the World Health Organization (WHO) considers nitrogen dioxide (NO 2 ) to be a serious pollutant [1].NO 2 is a significant source of global warming, haze, acid rain, and photochemical smog [2].Moreover, NO 2 has an impact on vegetation and crops by affecting plant growth efficiency and reducing crop yields [3].On the other hand, NO 2 is hazardous to human health, and high levels of NO 2 inhalation can cause severe health risks such as pulmonary edema, breathing difficulties, and bronchospasm [4].Long-term exposure to NO 2 increases the risk of high blood pressure [4].According to statistical analyses, each 10 µg/m 3 increase in NO 2 exposure increases all-cause mortality by 2%, acute lower respiratory disease by 6%, and chronic obstructive pulmonary illness by 3% [1,5,6].Therefore, the development of sensors responding to low concentrations of NO 2 is urgently demanded for improving the air environment and protecting human health.
Currently, the most common gas sensors are electrochemical sensors [7,8], solid electrolyte sensors [9], optical sensors [10,11], and semiconductor sensors [12,13].Semiconductor sensors are widely used in the detection of toxic and hazardous gases owing to their low cost, high sensitivity, and good stability [14].However, semiconductor sensors still have problems such as poor selectivity, high operating temperature, etc., which hampers their actual applications.
As a typical n-type metal oxide, SnO 2 has excellent physical and chemical stability, with a low cost and non-toxic characteristics, which makes it widely used in gas sensors [15].
In recent decades, researchers have been devoted to tackling the aforementioned problems via multiple approaches for metal oxide semiconductor (MOS) sensors, including geometric structure modification [16], elemental doping [17], heterostructure construction [18,19], and noble metal loading [20].Huang et al. prepared nanoflower-like Au/SnS 2 /SnO 2 heterojunctions using a solvothermal method and in situ decoration.The response value to 8 ppm NO 2 was 22.3 at 80 • C.These good gas-sensitizing properties were attributed to the formation of heterojunctions and the formation of more S vacancies, promoting more gas adsorption on the material surface [21].Mnrugesh et al. synthesized p-Co 3 O 4 /n-SnO 2 heterojunctions using a hydrothermal method.The prepared 10% Co 3 O 4 /SnO 2 had a response of 88% at 150 • C for 100 ppm NO 2 with good selectivity.The enhancement of the sensing properties was attributed to the formation of a potential barrier at the Co 3 O 4 /SnO 2 heterointerface, the high specific surface area, and the increase in oxygen vacancy content [22].
Unfortunately, disadvantages still exist with the above modification strategies.Doping inhomogeneous elements into the MOS matrix will change the original crystal structure and increase surface defects [23].The construction of heterojunctions via exogenous MOS or noble metals will increase the interfacial potential barrier, thereby increasing the baseline resistance and power consumption as well [24][25][26].All of the above methods require the introduction of other elements into the MOS matrix, which increases the preparation cost and makes the production process more cumbersome.Moreover, freeze-drying treatment does not form a gas-liquid interface during the whole process, and the capillary force does not cause structural collapse.During the freeze-drying process, the material is first cooled below its freezing point, where the moisture in it freezes to form ice crystals.The formation and growth of ice crystals exert physical stresses on the surrounding material [27,28].For semiconductor materials, this stress can lead to lattice distortions, which can introduce defects such as point defects, dislocations, and other defects, which, in turn, affect the electronic properties of the material.And the introduction of these defects can introduce new energy levels in the forbidden bands of semiconductors as trap energy levels or composite centers [29,30].Hitherto, fewer studies have been reported on pristine MOSbased material sensors through freeze-drying treatments.
In this work, SnO 2 nanoparticles were prepared using both a hydrothermal method and the following freeze-drying treatments.The results showed that the response value of SnO 2 -FD (886.2) to 10 ppm NO 2 at 100 • C was 17 times higher than that of SnO 2 -AD (52.5), with a shorter response recovery time (74/27 s) and a low detection limit (1.69 ppb).The effect of the drying method on their gas-sensitizing properties was systematically investigated.The small particle size of the nanoparticles allowed a larger area to be in full contact with the target gas, which provided more active sites for gas adsorption.The enhanced performance is also attributed to the increase in adsorbed oxygen and the improvement of electronic structure.Therefore, this study paves novel ways for developing high-performance MOS-based sensors.

Synthesis of SnO 2 Nanoparticles
SnO 2 nanoparticles were synthesized using a facile hydrothermal method.In total, 2.35 mmol SnCl 4 •5H 2 O and 10.8 mmol urea were dissolved in a mixed solvent with a volume of 17.2 mL deionized water and 2 mL absolute ethanol with 15 min magnetic stirring.Then, 2 mL ammonia was added to the above solution.After another 15 min of magnetic stirring, the mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave and was maintained at 200 • C for 14 h.The white products were collected and washed with deionized water and absolute ethanol.Two drying methods were employed to remove solvents.One involved drying the products obtained via centrifugation at 80 • C. The other involved rapidly pre-freezing the products in liquid nitrogen after aging them in deionized water for 1 day to improve the stability of the samples and to form a more homogeneous ice crystal structure.And then, the samples were further freeze-dried at −50 • C for 2 days.The white powders obtained using the two methods were calcined at 500 • C for 2 h and were, respectively, named SnO 2 -AD and SnO 2 -FD.

Material Characterizations
The crystal structure of the samples was analyzed by X-ray diffraction analysis (XRD, DMAX-2500 PC, Tokyo, Japan) with Cu-Kα (λ = 1.5418Å) from 10 • to 90 • with a scanning speed of 10 • /min.The chemical compositions and the valence state of elements were characterized via an X-ray photoelectron spectrometer (XPS, AXIS Supra, Manchester, UK) with Al-Kα (hν = 1486.6eV).The binding energy was calibrated using C 1s peaks at 284.8 eV.The morphology and microstructure of the samples were investigated by scanning electron microscope (SEM SU-70, Tokyo, Japan).The specific surface areas and pore size assignment of the samples were tested by a full-automatic specific surface and porosity analyzer (TriStar II 3flex, Micromeritics, Norcross, GA, USA) and separately calculated through Brunauer-Emmett-Teller (BET) and Barrett-Joiner-Halenda (BJH) methods.The electrical properties and carrier concentrations of the samples were measured by Hall Effect Measurement (HSM-5000, Seoul, Republic of Korea).The UV-vis spectra and band gaps of the samples were characterized via UV-vis diffuse reflection spectrum (Uv3600plus Shimadzu, Kyoto, Japan).The molecular structure of samples was analyzed by Raman spectroscopy (Thermo DXR2xi, Waltham, MA, USA) with a 1064 nm laser excitation.

Gas Sensing Performance Test
The gas sensors were fabricated using the prepared SnO 2 materials.First, the prepared samples were dispersed in deionized water with a mass of 1:5 and thoroughly ground in a mortar to form a homogeneous paste.The paste was applied to an Al 2 O 3 substrate with four electrodes printed on it and dried at 80 • C.This process was repeated five times to form a homogeneous sensitive film and heated in air for 10 h at 80 • C.Then, the substrates coated with the sensing layer were soldered to the pedestal and aged for one week at 3 V to ensure their stability.The gas sensing properties were measured with a WS-30B gas sensitivity instrument (Zhengzhou Winsen Electronics Co., Ltd., Zhengzhou, China).The target gases were injected into the test chamber via a syringe.Built-in fans in the test chamber rotated to bring the target gas into rapid and full contact with the sensor.R a and R g represent the stable resistance of the sensing material in air and after exposure to the target gas, respectively.The response value (S) is denoted by S = R g /R a for oxidizing gases and S = R a /R g for reducing gases.Response and recovery time are recorded as 90% time of total resistance changes in responding/recovering processes.

Characterizations
The crystal structure of SnO 2 was measured by XRD as shown in Figure 1a.All diffraction peaks of SnO 2 -AD and SnO 2 -FD are in accordance with the tetragonal structure of SnO 2 (JCPDS 41-1445).No other diffraction peaks appeared in the pattern, proving that the synthesized samples did not contain any other material phases.It can be observed that the SnO 2 -FD diffraction peaks are of higher intensity, indicating superior crystallinity compared to SnO 2 -AD [31].Increased crystallinity means fewer grain boundaries, which are the scattering centers of carriers since the arrangement of atoms on the grain boundaries is different from that inside the grains [32].On the other hand, grain boundaries are commonly accompanied by localized stresses and strains [33].Therefore, the reduction in grain boundaries reduces carrier trapping and scattering at grain boundaries, thus improving carrier mobility of SnO 2 -FD [34].The prepared SnO 2 grain sizes can be approximately calculated using the Debye-Scherrer equation as indicated in Equation (1) [35]: where λ is the wavelength of the radiation (1.5418 Å), β is the half-height width of the peak, and θ is the Bragg diffraction angle.The average grain sizes are 10.3 and 9.0 nm, corresponding to SnO 2 -AD and SnO 2 -FD samples.SnO 2 -FD has a smaller grain size, and its grain size is close to twice the Debye length of SnO 2 (3 nm) [36].As we know, when the grain size of aerogels is nearly twice the Debye length, the size of the grains affects their electrical conductivity, that is, they are more likely to be activated for some nanometer effects [37].Therefore, the depletion layer accounts for a large proportion of the particle volume, which is more favorable for exposing the SnO 2 active surface and thus exchanging electrons with the target gas.Thereby, the response value and the response/recovery speed of SnO 2 -FD can be improved [38].
The chemical compositions and the valence state of elements were characterized via XPS.As shown in Figure 1b, the Sn and O elements are identified in the wide spectrum.The Sn 3d XPS spectrum of SnO 2 is shown in Figure 1c, the two peaks at 486.52 eV and 495.03 eV corresponding to SnO 2 -AD are Sn 3d 5/2 and Sn 3d 3/2 , respectively [39].It can be observed that the Sn 3d 5/2 and Sn 3d 3/2 peaks of SnO 2 -FD are, respectively, shifted by 0.28 eV and 0.27 eV toward the high binding energy.Previous studies have shown that the total charge of an atom has a close influence on the chemical shifts of the peaks of the energy spectrum [40].The SnO 2 -FD binding energy displays a redshift, indicating that more electrons are captured by the O 2 molecules in air, resulting in a lower density of nearby electron clouds and an increase in the binding energy [41].Figure 1d, e shows the O 1s XPS spectra.The peaks of SnO 2 -AD located at ca. 530.4,531.8, and 533.4 eV correspond to lattice oxygen (O L ), oxygen vacancy (O v ), and adsorbed oxygen (O c ), respectively [24,42].It can be noted that the O c and O v contents of SnO 2 -FD are higher than those of SnO 2 -AD.The presence of O v can supply more electrons and promote the formation of adsorbed oxygen ions [43,44].On the other hand, O v disrupts the metal oxide integrity and provides more active sites for target gas adsorptions and gas-sensitization reactions [45][46][47].In particular, the increase in O c may promote an alternative gas-sensitive reaction pathway for NO 2 at the material surface [48][49][50].
The crystallography and structural features of SnO 2 -AD and SnO 2 -FD were investigated via a Raman system as shown in Figure 1f.The SnO 2 lattice typically generates the following major vibrational modes [51]: where A 1g , B 1g , B 2g , E g are Raman active modes, A 2u and E u are infrared active modes, A 2g and B 1u are inactive modes.The peak at around 633 cm −1 is assigned to the symmetric O-Sn-O vibration (A 1g ).The broadening of the A 1g peak of SnO 2 -FD indicates a reduction in its grain size [52].The Raman peak at around 484 cm −1 corresponds to the shear vibration of the oxide (E g ) [53].And the Raman peak at around 776 cm −1 is due to the asymmetric O-Sn-O stretching (B 2g ) [54].The Raman peaks of SnO 2 -FD all showed different degrees of blue shift, which might be caused by the increased content of oxygen vacancies [52].The appearance of these Raman peaks indicates the tetragonal structure of SnO 2 .The peaks near 249 and 306 cm −1 are inactive Raman modes, which can be attributed to localized structural disturbances [55].The enhancement of their strength is possibly due to structural defects introduced during the freeze-drying process.The morphology and microstructure of the samples were investigated by SEM as shown in Figure 2. It can be seen that both SnO2-AD and SnO2-FD are homogeneous nanospheres.It is indicated that the two drying methods have no significant effect on their morphology.The diameters of the SnO2 nanospheres are all approximately 10 nm, corresponding to the XRD results.This suggests that each SnO2 nanosphere is composed of a single crystal [56].Moreover, such a small particle size gives them a larger specific surface area for full contact with the target gas [57].The presence of abundant pore structures between the nanospheres further facilitates target gas diffusion.To further analyze the specific surface area and pore size distribution of SnO2, N2 adsorption-desorption tests were performed as shown in Figure 3.The N2 adsorption- The morphology and microstructure of the samples were investigated by SEM as shown in Figure 2. It can be seen that both SnO 2 -AD and SnO 2 -FD are homogeneous nanospheres.It is indicated that the two drying methods have no significant effect on their morphology.The diameters of the SnO 2 nanospheres are all approximately 10 nm, corresponding to the XRD results.This suggests that each SnO 2 nanosphere is composed of a single crystal [56].Moreover, such a small particle size gives them a larger specific surface area for full contact with the target gas [57].The presence of abundant pore structures between the nanospheres further facilitates target gas diffusion.The morphology and microstructure of the samples were investigated by SEM as shown in Figure 2. It can be seen that both SnO2-AD and SnO2-FD are homogeneous nanospheres.It is indicated that the two drying methods have no significant effect on their morphology.The diameters of the SnO2 nanospheres are all approximately 10 nm, corresponding to the XRD results.This suggests that each SnO2 nanosphere is composed of a single crystal [56].Moreover, such a small particle size gives them a larger specific surface area for full contact with the target gas [57].The presence of abundant pore structures between the nanospheres further facilitates target gas diffusion.To further analyze the specific surface area and pore size distribution of SnO2, N2 adsorption-desorption tests were performed as shown in Figure 3.The N2 adsorption- To further analyze the specific surface area and pore size distribution of SnO 2 , N 2 adsorption-desorption tests were performed as shown in Figure 3.The N 2 adsorptiondesorption isotherms of both SnO 2 -FD and SnO 2 -AD are of type IV mode with the type H2(b) hysteresis loop, indicating that both of them have mesoporous structures with similar hierarchical structures [58].The specific surface areas of SnO 2 -AD and SnO 2 -FD are ca.55.63 and 52.90 m 2 /g, respectively.The larger specific surface areas are attributed to the small particle size of SnO 2 nanoparticles.This large specific surface area supplies more active sites for the adsorption of the target gas, which is positive for the surface of the gas-sensitive reaction, thus shortening the response/recovery time of the sensors and enhancing the response value [59,60].As displayed in BJH measurement, the average pore sizes of SnO 2 -AD and SnO 2 -FD were calculated as ca.9.71 and 6.95 nm, respectively.The smaller pore size of SnO 2 -FD indicates the presence of smaller primary particles formed, tightly aggregating to form smaller mesopores [61].The pore size of the mesopore facilitates the adsorption and desorption of the target gas, thus effectively enhancing the gas sensing performance of SnO 2 [62,63].
desorption isotherms of both SnO2-FD and SnO2-AD are of type IV mode with the type H2(b) hysteresis loop, indicating that both of them have mesoporous structures with similar hierarchical structures [58].The specific surface areas of SnO2-AD and SnO2-FD are ca.55.63 and 52.90 m 2 /g, respectively.The larger specific surface areas are attributed to the small particle size of SnO2 nanoparticles.This large specific surface area supplies more active sites for the adsorption of the target gas, which is positive for the surface of the gassensitive reaction, thus shortening the response/recovery time of the sensors and enhancing the response value [59,60].As displayed in BJH measurement, the average pore sizes of SnO2-AD and SnO2-FD were calculated as ca.9.71 and 6.95 nm, respectively.The smaller pore size of SnO2-FD indicates the presence of smaller primary particles formed, tightly aggregating to form smaller mesopores [61].The pore size of the mesopore facilitates the adsorption and desorption of the target gas, thus effectively enhancing the gas sensing performance of SnO2 [62,63].

Gas Sensing Performance
The NO2 sensing characteristics of SnO2 sensors were investigated.The optimal operating temperature is an important indicator for evaluating the performance of gas sensors.Figure 4a shows the response of SnO2 sensors to 10 ppm NO2 under different operation temperatures.The response values of both SnO2-FD and SnO2-AD increase with increasing temperature and decrease after reaching a maximum at 100 °C.The reason is that the lack of thermal energy leads to gas adsorption that is weak or insufficient to overcome the energy barrier for gas-sensitive reactions at low temperatures, while the gas desorption rate is too fast for gas-sensitive reactions to occur at higher temperatures [64,65].The response value of SnO2-FD (886.2) to 10 ppm NO2 at 100 °C is about 17 times higher than that of SnO2-AD (52.5).
Figure 4b illustrates the baseline resistance variation of SnO2-FD and SnO2-AD at various temperatures, and it can be observed that the baseline resistance of the SnO2 samples decreases with increasing temperature, exhibiting typical semiconductor characteristics [66].Interestingly, the baseline resistance of SnO2-AD is about two magnitudes higher than that of SnO2-FD at the respective temperatures.This may be due to differences in carrier concentration.SnO2-FD has a higher concentration of carriers and therefore has a higher conductivity leading to a lower baseline resistance [67].On the other hand, as an n-type semiconductor, the response value (S) of SnO2 to the oxidizing gas NO2 is defined by the ratio of the stabilized resistance (Rg) exposed to NO2 to the baseline resistance (Ra) in air.A small baseline resistance causes a more significant change in resistance, resulting in a larger response value [68,69].The response/recovery curves of SnO2-AD and SnO2-FD sensors to 10 ppm NO2 at 100 °C are shown in Figure 4c,d.The response/recovery times of SnO2-FD are all shorter than those of SnO2-AD.In particular, the recovery time of SnO2-FD is 27 s profoundly lower than that of SnO2-AD (218 s), which is due to the increased

Gas Sensing Performance
The NO 2 sensing characteristics of SnO 2 sensors were investigated.The optimal operating temperature is an important indicator for evaluating the performance of gas sensors.Figure 4a shows the response of SnO 2 sensors to 10 ppm NO 2 under different operation temperatures.The response values of both SnO 2 -FD and SnO 2 -AD increase with increasing temperature and decrease after reaching a maximum at 100 • C. The reason is that the lack of thermal energy leads to gas adsorption that is weak or insufficient to overcome the energy barrier for gas-sensitive reactions at low temperatures, while the gas desorption rate is too fast for gas-sensitive reactions to occur at higher temperatures [64,65].The response value of SnO 2 -FD (886.2) to 10 ppm NO 2 at 100 • C is about 17 times higher than that of SnO 2 -AD (52.5).
Figure 4b illustrates the baseline resistance variation of SnO 2 -FD and SnO 2 -AD at various temperatures, and it can be observed that the baseline resistance of the SnO 2 samples decreases with increasing temperature, exhibiting typical semiconductor characteristics [66].Interestingly, the baseline resistance of SnO 2 -AD is about two magnitudes higher than that of SnO 2 -FD at the respective temperatures.This may be due to differences in carrier concentration.SnO 2 -FD has a higher concentration of carriers and therefore has a higher conductivity leading to a lower baseline resistance [67].On the other hand, as an n-type semiconductor, the response value (S) of SnO 2 to the oxidizing gas NO 2 is defined by the ratio of the stabilized resistance (R g ) exposed to NO 2 to the baseline resistance (R a ) in air.A small baseline resistance causes a more significant change in resistance, resulting in a larger response value [68,69].The response/recovery curves of SnO 2 -AD and SnO 2 -FD sensors to 10 ppm NO 2 at 100 • C are shown in Figure 4c,d.The response/recovery times of SnO 2 -FD are all shorter than those of SnO 2 -AD.In particular, the recovery time of SnO 2 -FD is 27 s profoundly lower than that of SnO 2 -AD (218 s), which is due to the increased porosity and the small particle size of SnO 2 -FD that promotes gas diffusion.To avoid errors due to serendipity, we performed two repetitive response recovery tests for SnO 2 -FD and SnO 2 -AD, respectively.As shown in Figure S1, the response/recovery times of SnO2-AD were 83/244 s and 96/202 s, whereas the response/recovery times of SnO 2 -FD were 71/43 s and 85/25 s, respectively.This indicates a significant improvement in the adsorption/desorption kinetics of SnO 2 -FD.porosity and the small particle size of SnO2-FD that promotes gas diffusion.To avoid errors due to serendipity, we performed two repetitive response recovery tests for SnO2-FD and SnO2-AD, respectively.As shown in Figure S1, the response/recovery times of SnO2-AD were 83/244 s and 96/202 s, whereas the response/recovery times of SnO2-FD were 71/43 s and 85/25 s, respectively.This indicates a significant improvement in the adsorption/desorption kinetics of SnO2-FD.The dynamic response curves of SnO2 sensors toward 0.1-15 ppm NO2 at 100 °C are shown in Figure 5a.The response values of SnO2-AD and SnO2-FD increase continuously with increasing NO2 concentration, and the response value of SnO2-FD is much higher than that of SnO2-AD over the entire concentration range.It can be observed that there is still a significant response of SnO2-FD to 100 ppb NO2. Figure 5b records the response of SnO2 sensors toward different concentrations of NO2 at 100 °C.The response value of the sensor is basically linear with NO2 concentration, indicating its potential capability of quantitative NO2 detection.Figure 5c is a magnified image of the NO2 concentration in the range of 0.1-2 ppm in Figure 5b.As can be seen from Figure 5c, the SnO2-AD sensor response values are all below 10 when the NO2 concentration is less than 2 ppm, whereas the SnO2-FD sensor still has a high response value toward a low concentration of NO2, which is still as high as 214.9 at 2 ppm.A linear fit is performed for the response values versus the concentration of NO2 in this range.The slope of SnO2-FD (116.7) is 25 times higher than the slope of SnO2-AD (4.7), indicating that the presence of a trace amount of NO2 can cause a variation in the response value.Moreover, the regression value (R 2 ) of SnO2-FD reached 0.975, indicating a favorable linearity, which is capable of providing The dynamic response curves of SnO 2 sensors toward 0.1-15 ppm NO 2 at 100 • C are shown in Figure 5a.The response values of SnO 2 -AD and SnO 2 -FD increase continuously with increasing NO 2 concentration, and the response value of SnO 2 -FD is much higher than that of SnO 2 -AD over the entire concentration range.It can be observed that there is still a significant response of SnO 2 -FD to 100 ppb NO 2 .Figure 5b records the response of SnO 2 sensors toward different concentrations of NO 2 at 100 • C. The response value of the sensor is basically linear with NO 2 concentration, indicating its potential capability of quantitative NO 2 detection.Figure 5c is a magnified image of the NO 2 concentration in the range of 0.1-2 ppm in Figure 5b.As can be seen from Figure 5c, the SnO 2 -AD sensor response values are all below 10 when the NO 2 concentration is less than 2 ppm, whereas the SnO 2 -FD sensor still has a high response value toward a low concentration of NO 2 , which is still as high as 214.9 at 2 ppm.A linear fit is performed for the response values versus the concentration of NO 2 in this range.The slope of SnO 2 -FD (116.7) is 25 times higher than the slope of SnO 2 -AD (4.7), indicating that the presence of a trace amount of NO 2 can cause a variation in the response value.Moreover, the regression value (R 2 ) of SnO 2 -FD reached 0.975, indicating a favorable linearity, which is capable of providing accurate concentration measurements.Furthermore, the good linearity simplifies data analysis [70].We can calculate the actual gas concentration from the response value of the output by the known linear equation fitted [59,71].The detection limit (LOD) of the sensor is predicted by Equation (3) [72]: where rms is the root mean square deviation of the baseline resistance and slope is the slope of the fitted line.The LOD of SnO 2 -AD is 127.53 ppb, and that of SnO 2 -FD is 1.69 ppb NO 2 .
And the regression value (R 2 ) of SnO 2 -FD amounts to 0.975, indicating a high reliability in practical applications.Figure 5d,e show the response/recovery time of SnO 2 sensors to NO 2 with different concentrations.It can be observed that the recovery time of SnO 2 -FD exposed to high NO 2 concentration is drastically shortened, and the response time is also reduced.The response/recovery time of a gas sensor is related to the diffusion rate of the gas and its surface reaction rate [73].Its response/recovery at low concentrations is dominated by the effect of the gas diffusion rate [74].The target gas concentration gradient at the sensor surface is quite resulting in a long response/recovery time [75,76].The responses of SnO 2 sensors to 10 ppm NO 2 , 10 ppm H 2 S, 100 ppm CO, 100 ppm HCHO, and 100 ppm ethanol at 100 • C are displayed in Figure 5f.The sensor is generally unresponsive to all gases except NO 2 , indicating that the sensor has excellent selectivity for NO 2 .The comparison of the performance of the SnO 2 -FD sensor in this work with the reported NO 2 sensor is shown in Table 1.Compared to the reported NO 2 sensor, the SnO 2 -FD sensor exhibits a high NO 2 response value (886.2) and a short response recovery time (74/27 s) towards 10 ppm NO 2 at 100 • C with an extremely low detection limit (1.69 ppb).
accurate concentration measurements.Furthermore, the good linearity simplifies data analysis [70].We can calculate the actual gas concentration from the response value of the sensor output by the known linear equation fitted [59,71].The detection limit (LOD) of the sensor is predicted by Equation (3) [72]: where rms is the root mean square deviation of the baseline resistance and slope is the slope of the fitted line.The LOD of SnO2-AD is 127.53 ppb, and that of SnO2-FD is 1.69 ppb NO2.And the regression value (R 2 ) of SnO2-FD amounts to 0.975, indicating a high reliability in practical applications.Figure 5d,e show the response/recovery time of SnO2 sensors to NO2 with different concentrations.It can be observed that the recovery time of SnO2-FD exposed to high NO2 concentration is drastically shortened, and the response time is also reduced.The response/recovery time of a gas sensor is related to the diffusion rate of the gas and its surface reaction rate [73].Its response/recovery at low concentrations is dominated by the effect of the gas diffusion rate [74].The target gas concentration gradient at the sensor surface is quite low, resulting in a long response/recovery time [75,76].The responses of SnO2 sensors to 10 ppm NO2, 10 ppm H2S, 100 ppm CO, 100 ppm HCHO, and 100 ppm ethanol at 100 °C are displayed in Figure 5f.The sensor is generally unresponsive to all gases except NO2, indicating that the sensor has excellent selectivity for NO2.The comparison of the performance of the SnO2-FD sensor in this work with the reported NO2 sensor is shown in Table 1.Compared to the reported NO2 sensor, the SnO2-FD sensor exhibits a high NO2 response value (886.2) and a short response recovery time (74/27 s) towards 10 ppm NO2 at 100 °C with an extremely low detection limit (1.69 ppb).6c.The increased humidity leads to a reduction in the resistance of the material, as shown in Figure S2.It is attributed to the reaction of water molecules with adsorbed oxygen species on the surface of the material to form hydroxyl groups and release electrons into the conduction band of the material [81].Moreover, the hydroxyl groups formed by water molecules can occupy the active sites on the material surface, which leads to metal oxide hydroxyl poisoning and inhibits gas adsorption [82,83].On the other hand, the reaction of water molecules with adsorbed oxygen on the surface of the material generates a competitive relationship with the reaction of NO 2 and adsorbed oxygen, which affects the gas-sensitive response of the sensor [84].They stabilize at relative humidity up to 40 RH% and SnO 2 -FD still has a higher response value (284.29) at 80 RH% compared to SnO 2 -AD (6.41).Figure 6d

Gas Sensing Mechanism
The gas sensing mechanism can be explained as the change in resistance of a semiconductor before and after exposure to a target gas, as shown in Figure 7.In air, oxygen molecules are adsorbed on the surface of the SnO2 sensor to capture its conduction band electrons to form reactive adsorbed oxygen species, resulting in an increase in SnO2 resistance [59].Upon exposure of the sensor to NO2, NO2 further traps electrons in the con-

Gas Sensing Mechanism
The gas sensing mechanism can be explained as the change in of a semiconductor before and after exposure to a target gas, as shown in Figure 7.In air, oxygen molecules are adsorbed on the surface of the SnO 2 sensor to capture its conduction band electrons to form reactive adsorbed oxygen species, resulting in an increase in SnO 2 resistance [59].Upon exposure of the sensor to NO 2 , NO 2 further traps electrons in the conduction band of SnO 2 due to its higher electron affinity than O 2 , leading to a further increase in its resistance [85].On the other hand, NO 2 reacts with adsorbed oxygen on the surface to form NO 2 − resulting in a decrease in the content of O 2 − , which further robs the electrons in the SnO 2 conduction band, leading to an increase in resistance [86].The SnO 2 -FD and adsorbed oxygen content is higher than that of SnO 2 -AD as shown in Figure 1d, which may also be reasonable for why the response value and the response/recovery rate of SnO 2 -FD are much higher than those of SnO 2 -AD at a high NO 2 concentration.In order to further understand the electrical properties, VH-I curves were tested using a Hall effect test system, as shown in Figure 8a,b, and the carrier concentrations were then calculated by Equation (4) [87]: where I is the excitation current, B is the magnetic induction, VH is the Hall voltage, and d is the material thickness.Here, VH/I can be expressed as the slope of the fitted straight line.The deviation of these dispersed points from the fitted straight line may be attributed to the non-uniform thickness of the coated gas-sensitive sensing layer, which results in a different concentration of electrons in each cross-section.During the measurement process, multi-point data were measured and fitted to minimize the error.The calculated carrier concentrations of SnO2-AD and SnO2-FD are 1.903 × 10 12 and 7.251 × 10 12 cm −3 , respectively.According to the XPS results, the content of O/Sn in SnO2-AD (1.76) is higher than that in SnO2-FD (1.60), indicating that the intrinsic defects of n-type SnO2 recombine with oxygen, thus leading to a lower carrier concentration in SnO2-AD [88].And the higher carrier concentration in SnO2-FD promotes a rapid gas sensing reaction [67].To have a better understanding of the energy band structure, the UV-vis diffuse reflectance spectra of SnO2-AD and SnO2-FD were tested, as shown in Figure 8c,d.In the visible light wavelength range, the absorbance of SnO2-FD is higher than that of SnO2-AD, indicating that more carriers can be produced in SnO2-FD [89].The UV absorption edge of SnO2-FD is redshifted; this is due to the straightforward electron transition between the valence bands In order to further understand the electrical properties, V H -I curves were tested using a Hall effect test system, as shown in Figure 8a,b, and the carrier concentrations were then calculated by Equation (4) [87]: where I is the excitation current, B is the magnetic induction, V H is the Hall voltage, and d is the material thickness.Here, V H /I can be expressed as the slope of the fitted straight line.The deviation of these dispersed points from the fitted straight line may be attributed to the non-uniform thickness of the coated gas-sensitive sensing layer, which results in a different concentration of electrons in each cross-section.During the measurement process, multi-point data were measured and fitted to minimize the error.The calculated carrier concentrations of SnO 2 -AD and SnO 2 -FD are 1.903 × 10 12 and 7.251 × 10 12 cm −3 , respectively.According to the XPS results, the content of O/Sn in SnO 2 -AD (1.76) is higher than that in SnO 2 -FD (1.60), indicating that the intrinsic defects of n-type SnO 2 recombine with oxygen, thus leading to a lower carrier concentration in SnO 2 -AD [88].And the higher carrier concentration in SnO 2 -FD promotes a rapid gas sensing reaction [67].To a better understanding of the energy band structure, the UV-vis diffuse reflectance spectra of SnO -AD and SnO 2 -FD were tested, as shown in Figure 8c,d.In the visible light wavelength range, the absorbance of SnO 2 -FD is higher than that of SnO 2 -AD, indicating that more carriers can be produced in SnO 2 -FD [89].The UV absorption edge of SnO 2 -FD is redshifted; this is due to the straightforward electron transition between the valence bands and conduction bands, suggesting that the decrease in the band gap of SnO 2 -FD reduces the activation energy of the electron transition [90].The band gap energies of SnO 2 -AD and SnO 2 -FD are ca.3.15 and 1.94 eV, respectively, indicating that the preparation of SnO 2 with the freeze-drying method significantly narrows the band gap.The reduction in the band gap may be due to the introduction of extensive defects [91].This reduces activation energy for electron migration and allows NO 2 to obtain electrons from the SnO 2 conduction band more efficiently, thus increasing its response value and response recovery rate [40,72].On the other hand, more electrons can be excited into the conduction band at a certain temperature, thus increasing the carrier concentration, which in turn promotes the electron transfer between the sensors and NO 2 that facilitates the gas-sensitized reaction.In this work, SnO2-FD has excellent NO2 sensing properties.First, the increase in chemisorbed oxygen content promotes an alternative reaction pathway for NO2 at high concentrations.Second, the SnO2-FD particle size is closer to the Debye length of SnO2, affecting its conductivity and facilitating the target gas contact with SnO2.In addition, SnO2-FD has a higher carrier concentration, which promotes electron exchange between the target gas and the sensing materials.Moreover, the band gap of SnO2-FD is drastically reduced, which lowers the activation energy of electrons transiting from the valence band to the conduction band and promotes the capture of electrons from the conduction band by the target gas, thus improving the response value of the sensor and the response/recovery speed.In this work, SnO 2 -FD has excellent NO 2 sensing properties.First, the increase in chemisorbed oxygen content promotes an alternative reaction pathway for NO 2 at high concentrations.Second, the SnO 2 -FD particle size is closer to the Debye length of SnO 2 , affecting its conductivity and facilitating the target gas contact with SnO 2 .In addition, SnO 2 -FD has a higher carrier concentration, which promotes electron exchange between the target gas and the sensing materials.Moreover, the band gap of SnO 2 -FD is drastically reduced, which the activation energy of electrons transiting the valence band to the conduction band and promotes the capture of electrons from the band by the target gas, thus improving the response value of the sensor and the response/recovery speed.

Conclusions
Small-sized SnO 2 -FD particles prepared by hydrothermal and freeze-drying methods have good gas-sensitive properties for NO 2 at lower temperatures.The SnO 2 -FD sensor exhibits an ultra-high response (886.2) with a short response recovery time (74/27 s) for 10 ppm NO 2 at 100 • C.Moreover, the sensor exhibits an extremely low detection limit, good selectivity, and humidity stability.The SnO 2 prepared by the freeze-drying method exhibits a significantly shortened band gap and increased carrier concentration, as well as a reduced particle size of SnO 2 particles.This study provides a new idea for research on semiconductor gas-sensitive material preparation methods.

Figure 4 .
Figure 4. (a) The response of SnO2 sensors to 10 ppm NO2 under different operation temperatures.(b) The resistance of SnO2 sensors at various operation temperatures.The response/recovery curves of (c) SnO2-AD and (d) SnO2-FD sensors to 10 ppm NO2 at 100 °C.

Figure 4 .
Figure 4. (a) The response of SnO 2 sensors to 10 ppm NO 2 under different operation temperatures.(b) The resistance of SnO 2 sensors at various operation temperatures.The response/recovery curves of (c) SnO 2 -AD and (d) SnO 2 -FD sensors to 10 ppm NO 2 at 100 • C.

Figure 5 .
Figure 5. (a) Dynamic response curves of SnO2 sensors toward 0.1-15 ppm NO2 at 100 °C.(b) The response of SnO2 sensors toward different concentrations of NO2 at 100 °C.(c) The linear relationship between response value and NO2 concentration from 0.1 ppm to 2 ppm for SnO2 sensors.(d) The response time and (e) the recovery time of SnO2 sensors in response to NO2 with different concentrations.(f) The response of SnO2 sensors to 10 ppm NO2, 10 ppm H2S, 100 ppm CO, 100 ppm HCHO, and 100 ppm ethanol.

Figure 5 .
Figure 5. (a) Dynamic response curves of SnO 2 sensors toward 0.1-15 ppm NO 2 at 100 • C. (b) The response of SnO 2 sensors toward different concentrations of NO 2 at 100 • C. (c) The linear relationship between response value and NO 2 concentration from 0.1 ppm to 2 ppm for SnO 2 sensors.(d) The response time and (e) the recovery time of SnO 2 sensors in response to NO 2 with different concentrations.(f) The response of SnO 2 sensors to 10 ppm NO 2 , 10 ppm H 2 S, 100 ppm CO, 100 ppm HCHO, and 100 ppm ethanol.

Figure
Figure 6a,b show the stability of SnO 2 and SnO 2 -FD sensors to 10 ppm NO 2 at 100 • C in five cycles.The response values of the sensors remain essentially unchanged over the five cycles, indicating the good reliability of the sensors.Ambient humidity is a factor that must be taken into account in the practical application of gas sensors.The response of SnO 2 sensors under different humidity levels to 10 ppm NO 2 at 100 • C is shown in Figure6c.The increased humidity leads to a reduction in the resistance of the material, as shown in FigureS2.It is attributed to the reaction of water molecules with adsorbed oxygen species on the surface of the material to form hydroxyl groups and release electrons into the conduction band of the material[81].Moreover, the hydroxyl groups formed by water molecules can occupy the active sites on the material surface, which leads to metal oxide hydroxyl poisoning and inhibits gas adsorption[82,83].On the other hand, the reaction of water molecules with adsorbed oxygen on the surface of the material generates a competitive relationship with the reaction of NO 2 and adsorbed oxygen, which affects the gas-sensitive response of the sensor[84].They stabilize at relative humidity up to 40 RH% and SnO 2 -FD still has a higher response value (284.29) at 80 RH% compared to SnO 2 -AD (6.41).Figure6dshows the response change of SnO 2 sensors to 10 ppm NO 2 at 100 • C for 30 days.The response values of the SnO 2 -FD sensors are generally stable over a period of 30 days with an average value of about 871.86, indicating favorable long-term stability.

17 Figure 6 .
Figure 6a,b show the stability of SnO 2 and SnO 2 -FD sensors to 10 ppm NO 2 at 100 • C in five cycles.The response values of the sensors remain essentially unchanged over the five cycles, indicating the good reliability of the sensors.Ambient humidity is a factor that must be taken into account in the practical application of gas sensors.The response of SnO 2 sensors under different humidity levels to 10 ppm NO 2 at 100 • C is shown in Figure6c.The increased humidity leads to a reduction in the resistance of the material, as shown in FigureS2.It is attributed to the reaction of water molecules with adsorbed oxygen species on the surface of the material to form hydroxyl groups and release electrons into the conduction band of the material[81].Moreover, the hydroxyl groups formed by water molecules can occupy the active sites on the material surface, which leads to metal oxide hydroxyl poisoning and inhibits gas adsorption[82,83].On the other hand, the reaction of water molecules with adsorbed oxygen on the surface of the material generates a competitive relationship with the reaction of NO 2 and adsorbed oxygen, which affects the gas-sensitive response of the sensor[84].They stabilize at relative humidity up to 40 RH% and SnO 2 -FD still has a higher response value (284.29) at 80 RH% compared to SnO 2 -AD (6.41).Figure6dshows the response change of SnO 2 sensors to 10 ppm NO 2 at 100 • C for 30 days.The response values of the SnO 2 -FD sensors are generally stable over a period of 30 days with an average value of about 871.86, indicating favorable long-term stability.Materials 2024, 17, x FOR PEER REVIEW 10 of 17

Figure 6 .
Figure 6.The stability of (a) SnO 2 -AD and (b) SnO 2 -FD sensors in response to 10 ppm NO 2 at 100 • C in 5 cycles.(c) The response of SnO 2 sensors under different humidity levels to 10 ppm NO 2 at 100 • C. (d) The response change of SnO 2 sensors in response to 10 ppm NO 2 at 100 • C for 30 days.

Materials 2024 ,
17,  x FOR PEER REVIEW 12 of 17 certain temperature, thus increasing the carrier concentration, which in turn promotes the electron transfer between the sensors and NO2 that facilitates the gas-sensitized reaction.

Table 1 .
The NO2 sensing performance of reported sensors and this work.

Table 1 .
The NO 2 sensing performance reported sensors and this work.